Mayor Mike McGinn said at a news conference those demonstrators got rid of their black clothing and rejoined the crowd. He said he had issued an order banning items at the demonstrations that would be used as weapons, the newspaper said.

Some Occupy protesters in Oakland came under tear gas attack Tuesday by police who arrested nine demonstrators, authorities said.

The Oakland rally was one of many that took place across the United States.

The nine taken into custody on suspicion of interfering with officers, failing to disperse and related counts were among more than 400 people who showed up at Oakland's Frank Ogawa Plaza near City Hall, the Los Angeles Times reported.

Police said they used "small amounts of gas" three times against "small groups of people who were committing violent acts," the newspaper said. Police said yellow paint was thrown at one officer, who was hit by a metal paint can and kicked in the ribs, the newspaper reported.

A television news van and a police van were vandalized, and there were other minor acts of vandalism and graffiti, authorities said.

Mostly, though, the rally was peaceful, the Times said. Among the protesters was Shaina Burnette, 31, who brought her 2-year-old son in a stroller decked with signs that read "Dear Corporate State, Can you please spare some clean air, water and food for my generation? Maybe a couple schools? Animal species? Trees? PLEASE…"

In Chicago, dozens of demonstrators blocked the entrance to a downtown Bank of America branch, police said.

More than two dozen city police officers were at the scene of the sit-in but no arrests had been made, WBBM-TV, Chicago, reported. About 15 of the protesters were ejected from the branch when they tried to go inside to open accounts, the TV station said.

Occupy demonstrators also joined forces with a larger group for a May Day march and rally in Union Park, the Chicago Tribune reported. Police officers patrolled the crowd's perimeter, and numerous police horses and riders were standing by for crowd control, the newspaper said.

The protests in Chicago come ahead of a May 20-21 summit of NATO countries' heads of state and government. The magazine Adbusters, which spearheaded the original Occupy Wall Street event, called Chicago "the focal point of this global spiritual insurrection" because of the summit.

In Philadelphia, about 125 Occupy protesters marched downtown, tying up traffic. Police warned them they would be arrested if they didn't disperse and Occupy protesters posted messages saying two people were arrested outside a bank, The Philadelphia Inquirer reported.

In Detroit, about 250 protesters, including members of the Occupy movement and the United Auto Workers, demanded immigration and education reform, among other issues as part of a May Day celebration.

"We have to be the change we want to see in this city," Katinah Milligan, 15, told The Detroit News.

Detroit police kept a watchful eye, though no conflicts had occurred by mid-afternoon, the newspaper said.

The Alliance for Immigrants Rights and Reform Michigan also was to hold a rally in Detroit Tuesday.

Meanwhile, authorities in New York said white powder found in envelopes sent to multiple banks in Manhattan Monday turned out to be cornstarch, WCBS-TV, New York, reported. The envelopes, which contained notes referring to "May Day" and made comments such as "You have 10 seconds to stop working," "You are not in control" and "[expletive] the banks," were sent to Wells Fargo banks, JP Morgan Chase headquarters and Mayor Michael Bloomberg's office.

Occupy movement marches, strikes and other acts of civil disobedience were planned in about 135 U.S. cities in an attempt to "disrupt the status quo," movement organizers said earlier.

"We call upon people to refrain from shopping, walk out of class, take the day off of work and other creative forms of resistance disrupting the status quo," organizers said in an e-mail Thursday.

Key cities targeted for the "general strike," or "a day without the 99 percent" -- which protest organizers said would be "the first truly nationwide general strike in U.S. history" -- were New York, California's San Francisco-Oakland area and Los Angeles, Chicago and Seattle, the Occupy Wall Street Web site said.

Marches were also planned for college towns such as Amherst, Mass., and Ann Arbor, Mich., the Web site said.

Occupy-related groups called for strikes and demonstrations in more than 80 countries, in cities including Amsterdam, Athens, Barcelona, London, Melbourne, Rome, Seoul and Toronto, the Web site said.

May 1 is already a date with strong historical significance in many countries, where it is a national holiday known as International Workers' Day, celebrated by working people and labor unions.

In the United States, May 1 is recognized as Law Day, to reflect on the role of law in the nation's foundation and to recognize its importance for society.

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